Avalanche photodiodes make it possible to obtain light sensors that have high sensitivity and high speed, e.g. for optical fiber transmission applications.
An avalanche photodiode comprises two main zones:
an absorption zone for absorbing photons, which zone is made of a material and has a thickness that depend respectively on the wavelength to be detected and on the speed of operation that is to be obtained; this zone is characterized by a relatively small forbidden energy band, in particular for applications at 1.5 .mu.m, and it is generally subjected to a moderate electric field; and PA1 a multiplication zone which enables internal gain to be obtained by means of the avalanche phenomenon; this zone can be of various compositions (uniform material or multilayer structure) and of various thicknesses, and it is subjected to an electric field that is very high. PA1 1! T. Kagawa, Y. Kawamura and H. Iwamura: "InGaAsP-InAlAs superlattice avalanche photodiode", IEEE Journal of Quantum Electronics, Vol. 28, No. 6, pp. 1419-1423, June 1992; PA1 2! I. Watanabe, S. Sugou, H. Ishikawa, T. Anan, K. Makita, M. Tsuji and K. Tagushi: "High-speed and low dark current flip-chip InAlAs/InAlGaAs quaternary well superlattice APD's with 120 GHz gain-bandwidth product", IEEE Photonics Technology Letters, Vol. 5, No. 6, pp. 675-677, June 1993; and PA1 3! S. Hanatani, H. Nakamura, S. Tanaka and T. Ido: "Flip-chip InAlAs/InGaAs superlattice avalanche photodiodes with back illuminated structures", Microwave and Optical Technology Letters, Vol. 7, No. 3, pp. 103-107, Feb. 20, 1994. PA1 4! Y. Le Bellego: "Photodiodes AlInAs/GaInAs pour transmissions optiques: composants passives a grande sensibilite et large bande passante", AlInAs/GaInAs photodiodes for optical transmission: passivated components having high sensitivity and large passband!, thesis at Caen University, 1991, with photodiodes in which the avalanche zone is made of a multiple quantum well AlInAs/GaInAs material, with the absorption zone being made of GaInAs. PA1 to obtain excellent separation between the electric field levels on either side of the transition zone; and PA1 to obtain conduction and valance bands that facilitate the passage of carriers from one to the other of the zones adjacent to said transition zone.
To separate the high electric field zone from the moderate field zone, it is common practice to provide an intermediate or transition zone between these two zones.
In the specific case of GaInAs/InP avalanche photodiodes, intended for photodetection in the 1.3 .mu.m to 1.6 .mu.m range, it is impossible to make the GaInAs material of the absorption zone operate at the high fields required for the InP material of the multiplication zone, so it is essential for these two zones to be separated.
By way of example, the transition zone can be constituted by a layer of InP or of AlInAs that is uniformly doped in order to obtain the field breakdown electric charge that enables a moderate electric field in the absorption zone to coexist with a strong field in the avalanche zone.
Avalanche photodiodes of this type (generally referred to as "Separate Absorption and Multiplication" or "SAM" avalanche photodiodes) are described, in particular, in the following publications:
Nevertheless, as can be seen in FIG. 1, such photodiodes suffer from discontinuities (of several tens of meV) in their conduction and valence bands on either side of the transition zone. This can give rise, in particular, to the carriers generated by photon absorption being captured or to ionization by impact, and it is harmful for photodiode performance since the ideal transport of carriers at high speed is disturbed. This effect is even more perceptible at low temperature.
More recently, avalanche photodiode structures have been proposed in which the transition zone has one or more layers of GaInAsP forming something which is misleadingly called a "gradual" or "graded" transition zone, and such photodiodes are said to be of the "Separate Absorption, Grading, and Multiplication" (SAGM) type.
This concept has been taken up and discussed in:
Because of the high ratio between the coefficients .alpha. and .beta. for ionization by electrons and by holes (.alpha./.beta..apprxeq.10), such photodiodes having a multiple quantum well avalanche zone are capable of presenting particularly satisfactory performance (high gain-bandwidth product) providing fields are available in the absorption region and in the multiplication region that are very uniform and well controlled.
In such structures, the passage of electrons from the absorption zone to the avalanche zone takes place via a transition zone comprising a first region of non-doped quaternary material of graded composition, and a second region of fixed composition (AlInAs) having uniform p+ doping (Be doping). In a variant, grading can also be internal to the doped region.
Such a transition zone makes continuous energy bands possible, with the level of doping in said transition zone determining field distribution in the photodiode.
Nevertheless, it suffers from the drawback firstly of not being very compact, and secondly of leading to reverse electric fields opposing the passage of carriers from one zone to the other.